The purpose of this invention is to provide a superior means of electrostatic shielding for power cables. It consists of an essentially full coverage longitudinally folded smooth or corrugated metallic tape which will permit expansion of the insulation and insulation shield, located directly under it, without significant deformation when the cable is at elevated temperatures corresponding to its normal, emergency and short circuit operating conditions. The tape may have a thin corrosion protective nonmetallic coating on one or both sides. This coating may be semi-conducting on one or both sides to permit the metallic shielding tape to accept charging current from the insulation structure while protecting certain metals, such as aluminum, against corrosion due to ingress of moisture into the cable.
The application of the metallic shielding tape so that it is longitudinally folded along the length of the cable further provides a permanent low resistance, low reactance path for voltage and current surges due to lightning or switching and for fault current regardless of whether the metallic tape is plain or coated for protection against corrosion. Circumferential corrugations in the tape as employed on cables of larger diameter facilitate bending of the cable during manufacture, installation and in training of the cable for splicing and terminating.
In the majority of cases the shielding tape will take the form of a longitudinally folded and overlapped corrugated copper or aluminum shielding tape. In the latter case the aluminum will have a firmly bonded semi-conducting layer of a polyethylene or polyvinyl chloride based compound on the side of the tape facing the semi-conducting insulation shield. The other side of the tape may have the same coating of an insulating coating of the same type compound and perhaps treated or with a supplementary coating to facilitate bonding to the overall jacket. In the case of the copper shielding tape, a longitudinally applied bridging tape over the exposed edge of the metallic tape with or without a binder thread may be employed to prevent the edge of the metallic tape from cutting into the jacket. The longitudinally folded tape shield may be applied in the same operation as the overall jacket or in a separate operation.
This invention may be employed in single conductor cable which may be shipped as such or factory-cabled into an assembly of two or more single conductor cables or in multiple conductor cable with an overall covering.
This method of shielding is superior to that which is presently commonly used whereby a metallic tape, normally plain or tinned copper, is helically applied overlapped on itself, by approximately 10 to 25 percent of its width, over the insulation structure (including semi-conducting insulation shield) of the cable. With this type of presently used shield, uneven expansion of the insulation structure of the cable occurs at elevated temperatures under normal and particularly under emergency and short circuit operating conditions, due to the reinforcing action of the double thickness of tape at the overlaps. As a consequence thereof, at elevated temperatures the insulation and insulation shield are significantly deformed such that they take on the surface contour of an interlocked or BX armored cable. This severe deformation may adversely affect the electrical properties and physical integrity of the insulation and may seriously increase the resistivity of the insulation shield, leading to premature failure of the cable.
With this presently used method of metallic tape shield application, it is not practical to make use of low-cost aluminum metal. Uncoated aluminum is not acceptable for the purpose because it is highly susceptible to corrosion on ingress of moisture into the cable. The application of a thin layer of semi-conducting or insulating material to one or both sides of the aluminum tape to protect it against corrosion will result in a very high resistance, high reactance path when the tape is helically applied on the cable forming a short lay helical path for voltage surges and fault currents.
Both of the adverse conditions described above are corrected by this invention in which the tape is longitudinally folded rather than helically applied. In addition, longitudinally folding the tape around the cable provides a permanently lower resistance and reactance shield path for the same amount of metal as when the tape is helically applied and the cable is subjected to aging and load cycling in service. A very important advantage is that longitudinally folding the metallic shielding tape permits the use of a corrosion-protected low-cost aluminum tape which, by virtue of a firmly bonded semi-conducting layer, will permit the aluminum shield to accept charging currents from the insulation structure of the cable.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.